Fatty acids and their derivatives, e.g. fatty acid alkyl esters, are commercially valuable as chemical intermediates as well as fuels. For example, fatty acid methyl esters (FAME), derived from vegetable oils, are used as an alternative to fossil-derived diesel fuels. Currently, the majority of biodiesel is produced via base-catalyzed trans-esterification of triglycerides contained in natural oils. However, the conversion of fats or oils that may otherwise be used for food or animal feeds is not desirable. Soaps (salts of fatty acids) that are produced during the vegetable oil refining process represent an alternative source of free fatty acids and do not compete with food or animal feeds.
Crude vegetable oils typically undergo several refining steps to remove fatty acids and other impurities. During alkali refining, the standard refining method for crude soybean and other vegetable oils, fatty acids present in the crude oils are neutralized using a base, e.g. sodium hydroxide, and separated from solution in an aqueous form. The separated product, commonly referred to as “soapstock” comprises primarily soaps, as well as varying levels of other lipid products including mono-, di- and triglycerides, un-neutralized fatty acids, and phosphatides.
Several methods for processing soapstock into free fatty acids or alkyl esters are known in the art. Some of the prior approaches rely on the use of strong acids, such as sulfuric or hydrochloric acids, to acidify the soaps to generate free fatty acids. Other approaches involve the use of an acidic resin and moving bed chromatography to generate fatty acid alkyl esters from soaps. The techniques in the prior art for generating fatty acids and fatty acid derivatives rely on the use of expensive and/or environmentally degrading chemicals, complex reactions requiring long residence times, or some combination thereof. Because soapstock is only available in relatively small quantities relative to the volumes of oil processed during vegetable oil refining, an economically efficient method for converting soapstock into free fatty acids and derivatives thereof should use inexpensive, widely available chemical reactants and mild process conditions.